1. Field of the Invention
The invention relates generally to motors which provide both linear and rotary motion. More particularly, the invention relates to a combined linear-rotary direct drive step motor that is capable of providing both unrestricted rotary motion and stepped linear motion over a predefined range, for example, along a portion of the longitudinal axis of the rotor shaft.
2. Description of the Related Art
Often in robotics applications it is necessary to provide both rotary and linear motion to an end effector (e.g., vacuum pickup, etc.) mounted on a robot arm. These dual motions are typically obtained by use of some combination of motors, linkages and hydraulic or pneumatic cylinders, etc. The closeness of these movements to the object increases resolution, sensitivity, and the speed of the operation. It would greatly reduce the complexity and size of the mechanism if the rotary and linear functions could be combined in a single compact housing mounted at the end of the robot arm to provide direct drive positioning of the end effector.
Many techniques and devices are known for providing either rotary or linear motion (but not both in combination using a single housing) as, for example, those described in U.S. Pat. No. 3,441,819 to A. Palmero; and U.S. Pat. No. 3,453,510 to K. G. Kreuter et al.
Palmero describes a reciprocating linear motor, utilizing a solid armature with a laminated stator, for producing small, discrete linear steps. The techniques described for producing linear motion rely on the rotation of a rotor cooperating with a stator in which both the rotor and stator are formed with teeth, with the teeth of one being in radial planes and the teeth of the other lying on a helical path. Energization of the stator causes a linear movement of the rotor.
The helical threads required by Palmero, prevent the use of a laminated rotor. This is problematic because high eddy current losses are likely to result from the use of a non-laminated rotor. Such losses affect the ability to operate stepper motors at high speeds.
The Kreuter et al patent separately describes both linear and rotary motors. The linear motor utilizes helical teeth cut in the stator. The rotor and stator are solid (not laminated). Fabrication of the motor is relatively expensive when compared to motors fabricated from laminations and the eddy current loss problem referred to hereinabove remains unsolved.
Karidis and Pawletko, in an article entitled "A Radial-Pole Linear Reluctance Motor", teach a variable reluctance motor which utilizes a complex double helix arrangement for providing linear motion only. The linear motor utilizes a laminated armature assembly which is physically similar to the stator assembly of a conventional rotary variable reluctance motor to allow for economical, high volume production.
The stator bar for the motor taught by Karidis and Pawletko consists of a round shaft or tube having a double helical groove on its surface. External roller bearings of conventional linear motor designs are eliminated and replaced with sliding contact between the armature assembly and the stator shaft.
However, the double helical groove arrangement taught by Karidis and Pawletko is difficult to fabricate. Additionally, although the armature assembly is laminated, the rotor taught in the reference is of necessity solid (e.g., not laminated) further subjecting the motor taught to the aforementioned high eddy current losses, etc.
Prior art mechanisms are also known that do provide both rotary and linear motion in a single housing. For example Kant, in U.S. Pat. No. 4,197,488, describes a motor that combines the desired stepped linear and unrestricted rotary motions utilizing a single housing. However, the aforementioned problems relating to eddy current loss and fabrication cost are not solved by Kant. Neither the stator or armature used by Kant are formed from laminations (helical teeth are used).
Kelby et al, Jr. in U.S. Pat. No. 3,745,433, describes a positioning mechanism capable of providing such motion. The positioning mechanism described utilizes an electrodynamic device (a moving coil), rather than an electromagnetic device. It is not a stepping motor, where positions are controlled by a number of pulses, and so encoding of both the rotary and linear positions is required. This is accomplished by the use of potentiometers that are coupled to the drive by rack and pinion gears.
A direct drive stepper motor based mechanism in any motor that provides the desired combination of rotary and linear motion would be effective in eliminating the need for the aforesaid encoding, potentiometers, rack and pinion gears, etc.
B. A. Sawyer, in U.S. Pat. No. 3,376,578 describes a two axis (x-y) positioning device for driving chart plotters and other devices. The device comprises a marker carrying head floated on an air bearing over a platen with no mechanical interconnections. A plurality of electromagnets in the head and means for selectively energizing the electromagnets to move the head across the magnetic platen, and along both axes simultaneously if desired, is described.
Sawyer teaches the use of a grid of non-magnetic material enclosing zones of magnetic material in the platen with the grid defining a first axis and a second axis in the plane of the surface over which movement is desired.
Although the Sawyer device is electromagnetic, utilizing stepper motors to provide motion in a plane; it does not provide the desired degrees of freedom to permit the desired combination of both rotary and linear motion. Additionally, the particular "waffle board" arrangement of the platen taught by Sawyer is complex to manufacture and is non-laminated, increasing the potential, for the aforementioned high eddy current losses, lower maximum stepping speed, etc.
An article by Higuchi et al, published in the Official Proceedings of the Twelfth Annual Motor-Con '88 Conference, June 1988, teaches a magnetic suspension type motor element which provides unrestricted rotary motion. A form of linear motion is also provided (a linear thrust).
Higuchi et al's rotor element is thrust (not stepped) longitudinally along the axis of the rotor shaft to essentially a fixed linear position (i.e., a position from which it cannot be linearly stepped). The rotor element is magnetically suspended in a fixed radial position in order to centrally locate the shaft. Accordingly, although Higuchi et al does teach a motor that provides a form of linear motion (thrust) combined with unrestricted rotary motion, the desired linear stepping capability, over a predefined range, combined with unrestricted rotary motion, is not taught.
In view of all of the above, it would be desirable to provide a combined linear-rotary direct drive step motor that can produce unrestricted rotary motion in combination with stepped linear motion over a predefined range. Furthermore, it would be desirable to be able to fabricate such a motor in a single compact housing utilizing a laminated core to minimize the aforementioned eddy current loss and stepper motor speed problems, and keep fabrication costs to a minimum.